The data comes from the Atlas3D survey http://www-astro.physics.ox.ac.uk/atlas3d/ which included approximately 800 early- and late-type galaxies. In the plot, the distribution of the galaxies in the sky can be seen. Sky coordinates use right ascension and declination instead of longitude and latitude.

Galaxies can be classified according to their appearance or by parameters such as the presence of a disk of stars, compactness of spiral arms, or smoothness of the light distribution. In the Atlas3D survey, the galaxies were classified into ellipticals and spirals by visual inspection.
The color of the markers in the plot shows the T-type, which is used to classify galaxies into elliptical (T <= -0.5, early-type) and spiral (T > -0.5, late-type). The color bar shows the range of values for the plotted galaxies. The lowest possible value is -5 and the highest possible 10.

An elliptical galaxy.

A spiral galaxy.

The color bar
showing T-type.

With the slider you can select which galaxies to plot according to heliocentric velocity relative to the barycenter of the Solar System. For example, to see the center of the Virgo cluster, set the minimum to -300 and the maximum to 200.

Plot showing the center of the Virgo cluster.

Clicking on a marker shows the information for that galaxy below the plot. If the galaxy was observed by the Sloan Digital Sky Survey, the URL will take you to its Navigate section.

The galaxy in the Sloan Digital Sky Survey.